The ubiquity of headphone ports makes them an attractive option for use as communication channels for attachable hardware devices. For example, 3.5 mm audio ports are standard on many devices, and in particular mobile devices such as smart phones and tablet computers. Such ports can be used to provide communication between a host mobile device and a hardware attachment, such as an attachable card reader (e.g. “reader”) for reading information from the magnetic stripe of a payment card.